Polarimeters measure the state of polarization of an input optical signal. Measurement of light polarization and its variation in time is important for many photonic applications, including telecommunications and fiber sensors. The transfer properties of fiber-based optic devices such as isolators, couplers, amplifiers and the like depend on the polarization state in the fiber itself. Thus, to completely characterize these devices, the relationship between the input and output states of polarization (SOP) of the fiber-based system must be known.
A conventional method for measuring the SOP of a light beam includes aligning a waveplate and a linear polarizer in the optical path of the beam. The waveplate is rotatable about the optical axis and is typically a quarter-wave plate. An optical sensor, such as a photodetector, is positioned to measure the intensity of light transmitted by the waveplate and polarizer. In operation, the waveplate is sequentially rotated to a minimum of four angular positions about the optical axis relative to the linear polarizer, and the transmitted light intensity is measured at each position by the photodetector.
The art has led to the development of in-line fiber polarimeters that utilize a set of four solid-state detectors that absorb only a small, scattered portion of a propagating optical signal, allowing for the remainder of the signal to continue along and impinge each detector in turn. Each detector develops an electrical signal proportional to the polarization-dependent fraction of light that it absorbs from the fiber. The four electrical output signals are then used to determine the four Stokes parameters of light in the fiber via an instrument matrix determined by calibration (at times, referred to as a “calibration matrix”).
The four Stokes parameters S0, S1, S2 and S3 (which combine to form the “Stokes vector”) are generally defined as follows: S0 is the total power, S1 is the linearly-polarized horizontal component minus the linearly-polarized vertical component, S2 is the linearly-polarized component at 45° minus the linearly-polarized component at −45°, and S3 is the right-hand circularly polarized component minus the left-hand circularly polarized component. Thus, a 4×4 calibration matrix C can be developed to define the relationship between a set of four detector output signals D1, D2, D3, D4 and the four Stokes parameters S0, S1, S2, S3:
            [                                                  S              0                                                                          S              1                                                                          S              2                                                                          S              3                                          ]        =                  [                                                            C                00                                                                    C                01                                                                    C                02                                                                    C                03                                                                                        C                10                                                                    C                11                                                                    C                12                                                                    C                13                                                                                        C                20                                                                    C                21                                                                    C                22                                                                    C                23                                                                                        C                30                                                                    C                31                                                                    C                32                                                                    C                33                                                    ]            ⁡              [                                                            D                1                                                                                        D                2                                                                                        D                3                                                                                        D                4                                                    ]              ,where the Stokes vector S is the sum of the four parameter components.
Classic polarimeters such as those described above are designed to accurately measure the time-averaged state of polarization (SOP) of the light source. When the degree of polarization (DOP) is 100%, the time-averaged polarization will be seen as the instantaneous polarization. For instance, when a signal has a sufficiently narrow linewidth, the time-averaged SOP will be the same as the instantaneous SOP. In most cases, however, optical signals will exhibit a finite bandwidth that extends over a range such that the polarization of the signals varies over their spectral bandwidth. Therefore, at any point in time, the “actual” (instantaneous) SOP may differ from the measured average value. Equivalently, their polarization can vary in time at a rate that is too great for conventional detectors to track. In this case, the DOP will be less than 100%. It has been found that if the original optical signal has a sufficiently large bandwidth, it is possible that the value of the Stokes vector S will vary across the spectrum of the optical signal. In this case, the DOP as measured from the SOP may be less than unity at any given time.
FIG. 1 is a Poincare sphere representation of the average Stokes vector S, showing a variation that may exist across the spectrum, where this variation is also referred to as a spectral “string” s of polarization states or, simply, “SOP string”. The set of polarizations forming the SOP string s may also be referred to as the SOP of the signal, in contrast to the “average SOP”. The bold arrow representation of the average Stokes vector S is the frequency/time-averaged vector measured by a standard polarimeter. A measurement of the Stokes vector for an exemplary single frequency component ω is represented as Sω in FIG. 1. It is to be noted that a corresponding string s that varies as a function of time (instead of frequency) may also be depicted for a time domain representation of the Stokes vector, which is derived from a Fourier transformation of the spectral data into temporal data.
While a conventional polarimeter can measure the average state of polarization, it cannot measure rotations of the SOP string about the time average SOP. If the polarization state of the propagating optical signal changes as a function of time, it is possible for the averaged Stokes vector S to remain fixed (measured, for example, by using rotating half-waveplates and quarter-waveplates), while the underlying spectral string rotates about this average value. FIG. 2 is a Poincare sphere representation of this time-dependent characteristic, illustrating the rotation of the exemplary string s through an angle α on the sphere to position s′, while the Stokes vector S remains constant (the shape of the spectral string also remains fixed for the case of FIG. 2).
In the case of either a relatively large bandwidth or large time-dependent changes in polarization (or both), information about the rotation of the spectral (and/or temporal) SOP string with respect to the average SOP will not be recognized by a conventional polarimeter, rendering the measurements incomplete. It would be desirable to add a functionality to a polarimeter so as to make it sensitive to the polarization transformations that cause the SOP string to rotate about the average SOP value.
Prior art spectral polarimeters have been developed to provide a measurement of the Stokes vector as a function of wavelength for wideband applications. In these arrangements, wavelength filtering (or the use of polarization sensitive elements with sufficient wavelength dispersion) is required to be used so that the polarization properties may be recorded as a function of wavelength. However, such measurements require complex dispersive elements, movable polarization elements, large detector arrays and/or slowly scanning filters, all of which make the resulting device large, slow, costly and complicated. Moreover, such spectral polarimeters are not necessary to provide information with respect to rotations of the SOP about the average SOP. A need remains, therefore, for a more efficient and compact, as well as higher speed, arrangement to provide information regarding the rotation of the SOP string with respect to average SOP.